1. Field of the Invention
The present invention relates to semiconductor devices. Alternatively, the present invention relates to wireless tags (also referred to as RFID tags, IC tags, or ID tags) utilizing a radio frequency identification (RFID) technology.
2. Description of the Related Art
RFID is widely used as an automatic identification technology by which data can be transmitted and received wirelessly. RFID includes an RFID tag which can store data and a reader/writer which performs reading of data stored in the RFID and writing of data to the RFID tag. An RFID tag includes, for example, a semiconductor device including an antenna and the like.
An RFID tag is provided with a session flag (Patent document 1). The session flag prevents additional reading of data after the data is read from the RFID tag. For example, the session flag has two kinds of states of “A” and “B” and the state is set from “A” to “B” when data is read out.
After data is read and the state of a session flag is set from “A” to “B”, a memory provided in the RFID tag holds the state of the session flag even when an electric wave is temporarily interrupted. Even if the RFID tag receives an electric wave, the RFID tag is “already read out” because the session flag is in the state “B.” Therefore, additional reading of the data stored in the RFID tag, which has already been read out is prevented.
Usually, a memory includes a transistor and a capacitor, and charge is accumulated in the capacitor to store data. However, charge accumulated in a capacitor leaks due to various causes. A major cause of leakage current is quantum tunneling. Accordingly, refresh operation is needed. Note that refresh operation is operation in which data is rewritten to a memory by accumulating charge on the basis of data which is stored in the memory and read out from the memory before charge is completely lost.